Recording on optical media is achieved by modulating a laser spot, focused on the active layer of the media, with a time function. Typically the time function is digital, generated by a data encoder, and the laser is pulsed from a relatively low read power to a higher level write power. The desired effect is that the media is "marked" where the laser was pulsed and blank elsewhere. The physical marking mechanism of the media is generally a function of temperature. The temperature, in turn, is a function of laser light intensity, and exposure time. The laser spot has a finite size, and an approximately gaussian shape. The exposure time is dependant upon media velocity and the modulating time function. It is possible to show that the energy distribution on the media surface is the convolution integral of the modulating time function and the laser spot. An adiabatic thresholding media is "marked" where this energy distribution exceeds the threshold, and unchanged elsewhere. Due to the finite spot size and the effects of the convolution process, the region on the media surface which exceeds the threshold (and is marked) is not only dependent on the modulating time function, but also on the shape and amplitude (or write power) of the laser spot. On read-back, the marked and unmarked regions of the media are detected in the intensity of the reflected laser light, which is directed to a photodiode by the optics. The output of such a device is an analog signal, from which the desired digital waveform must be reconstructed. The circuitry, which performs the recovery of the digital waveforms, usually includes some form of amplitude threshold qualification to eliminate spurious transitions due to noise in the signal. This thresholding requires that the analog read-back signal has a fairly constant amplitude. The amplitude of the analog read-back signal is dependant upon a long list of things, including the mark length, width, and on some media, depth. All of which are dependant upon the write power which was used to record the marks in the first place.
Generally, the modulating time function is the output of a digital data encoder. As such, it conforms to very specific timing rules. The write/read process may deterministically distort this type of signal to some degree. Frequently, the original digital encoder output is subjected to some pre-compensation, or deliberate distortion prior to recording, such that upon read-back, the write/read process distortions are partially cancelled, and a better likeness of the original ideal time function is recovered. One simple pre-compensation technique, referred to herein as pre-emphasis, is the delaying of all falling edges of the digital waveform by a constant time. The result is that all write pulses are lengthened and all spaces are shortened.